The present invention relates to a foil trap device for debris mitigation comprising a plurality of spaced apart foils extending from an entrance side towards an exit side of the foil trap device, said foils being arranged to allow a straight pass of radiation between the entrance side and the exit side of the foil trap device.
In systems with extreme ultraviolet (EUV) and/or soft X-ray radiation optical components such as mirrors or filters are in use which are arranged close to the high power light source. Both, optical components and light source, are in vacuum or least at a very low pressure level (typically 1 to 100 Pa) in order to minimize the absorption of the target radiation to be exploited. The present invention in particular relates to such EUV lithography systems.
In general, the light source, e.g. a pulsed plasma light source, is not merely emitting photons but also producing undesired material, so called debris, such as for example Sn, Li, Sb or Xe depending on the nature of the source itself. The debris defined in this sense may condense on the optical components whose performance consequently deteriorates, and a status may result, in which the optical component is inefficacious after a relatively short time. Due to the high plasma temperature a part of this debris consists of very fast ions and atoms in the keV range of energy. Apart from the very fast debris material not considered here also remarkably high amounts of slower atoms, ions and material droplets as well as particles are to be expected along the optical ray path. This type of debris is detrimental to the properties, e.g. EUV reflectivity, of the optical components, mainly due to deposition of material on the surface of the optical components. It is estimated that without any protection the lifetime of the optical components only due to deposition of material will be less than 104 shots of the pulsed light source. End of lifetime is defined by a 10% EUV reflectivity loss of each mirror component in use.
In order to protect the optical components from contaminating debris material in the aforementioned sense, in state of the art EUV lithography systems, in particular in the source containing devices, usually a so called foil trap is implemented. The foil trap is a debris mitigation tool that is located between the EUV light source and the optical components. The foil trap consists of a plurality of thin foils, also called blades or lamellas, with a thickness of e.g. 0.25 mm. These foils are typically arranged in parallel or in a radial way and spaced apart from each other by for example 1 mm, becoming larger at the outer circumference in case of a radial arrangement. A foil trap having the foils radially arranged with respect to a rotational axis may be operated with high rotational frequencies to trap high velocity particles. Such a foil trap is disclosed for example in WO 2006/134512 A2. In addition, the device is generally applied with an inert buffer gas. This leads to a deceleration of particles and material, and in conjunction with e.g. a rotational motion of the foils around the rotational axis, contaminants are collected on the walls of the foils of the trap and thus cannot reach the optical components. In EUV radiation systems with a wavelength of 13.5 nm mainly Argon is used as buffer gas, because it is an optimum compromise between effective deceleration of particles and absorption losses of EUV radiation. Other gases such as for example He, Ne or Kr, can be used, too. A large variety of foil trap designs based on the mentioned principles exist. All are designed for high EUV radiation throughput.
One disadvantage of current foil traps relates to the future perspective of high thermal loads expected in upcoming generations of EUV sources, especially of high-volume production tools for EUV lithography. It can be foreseen that the contaminant trapping device has to deal with several kW of heat at dedicated regions of the device. This is due to the ever increasing input power of several ten kW in future systems and thus, a corresponding increased demand in heat dissipation. The heat fraction stored in and delivered by particles and radiation probably may sum up to as high as 50% of the input energy. It is thus clear that the optical components as well as the foil trap have to be designed properly to be able to withstand this energy input. The heat load to the foil trap has a geometric dependence on design originating from azimuthal and radial distances to the EUV source delivering the heat. Furthermore, the heat conduction of the foil trap is dependent on the number and thickness of the foils but also on the heat conductivity and heat diffusivity of the material of the foils at elevated temperatures of e.g. 800 K or more. It is not straightforward that current and future designs applying current state of the art materials may result in systems being capable of dealing with these high temperatures and fulfill dedicated heat dissipation schemes in order to guarantee a stable and continuous operation and a long lifetime.
Another disadvantage of most current foil traps refers to the thermo-chemical properties of the materials in use. In all state of the art systems the materials of choice are preferably metals like copper, tantalum, molybdenum, tungsten or steel. The problem is that at elevated temperatures every oxide-free metal or alloy will likely react with the still hot Sn delivered from a Sn based plasma light source towards the foil trap. This chemical reaction, also considered as a form of corrosion by Sn, will deteriorate the properties of the metal based foil trap. As long as there is no saturation effect of reaction, the foil trap material will dissolve completely and thus lead to the total failure of the debris mitigation. Ceramic materials may be a solution for this problem. However, these ceramic materials suffer from the problem to deal with the above mentioned high thermal loads, as they usually exhibit only a very low thermal conductivity. A further possibility is to use foil traps made of a carbon-fibre composite material, as described in WO 2009/035328 A1. The carbon-fibre composite material provides a higher resistance against Sn.